1. Field of the Invention
The present invention relates to disk drives; more particularly, to high performance disk drives having multiple actuators.
2. Description of the Related Art
Conventional disk drives for use in work stations, personal computers, portable computers, and lap top computers, utilize a single actuator to position a single head with respect to each disk surface. Such disk drives often incorporate a plurality of disks organized in a stack, and a plurality of heads associated with corresponding ones of the disk surfaces. All of the heads are supported and positioned by the single actuator. During operation, only a single head reads or writes information from or to each disk surface.
Greater demands are being placed on disk drives by (1) the use of multi-user and/or multi-tasking operating systems, (2) work stations which provide an operating environment requiring the transfer of large amounts of data to and from a hard disk and/or large numbers of disk accesses to support large application programs or multiple users, and (3) the continuing trend toward higher performance microprocessors. The demands on disk drives include higher data transfer rates along with greater access to data.
To satisfy these demands, it has become desirable to increase the performance of hard disk drives beyond the performance levels which can be achieved by reducing access times of single actuator disk drives. (The access time of a disk drive is conventionally known as the average seek time determined by dividing the total time required to seek between all possible ordered pairs of track addressed by the total number of ordered pairs addressed.) Although access times are one factor which determines drive performance, other factors, including the data transfer rate (both to and from the media and to and from the internal buffer of a disk drive), the overhead imposed by the electronics of the disk drive, and the average latency time, all effect the performance of a disk drive.
Conventional disk drives having a single actuator provide performance levels limited by the following factors: (1) Only one track on any one data surface can be accessed at a given time; as a corollary, considering all of the heads, only one cylinder can be accessed at a given time (a cylinder is a vertically oriented segment representing the same track on the surfaces of all of the disks). (2) It is difficult, if not impossible, to simultaneously operate any two heads mounted on a single actuator. (3) In a multi-user or multi-tasking environment one process which is a disk intensive activity substantially reduces the ability of any other process to access data.
One conventional approach to enhancing access to data is to construct a disk drive using the so-called "head-per-track" approach, where one head is dedicated to each track on each disk surface. This approach is not practical for the large track densities (on the order of 1,000 or more tracks per inch) where thousands of heads would be required for each disk surface. In the environment of a work station, personal computer, or portable computer, the electronics required to operate such a large number of heads and the space required for the structural components necessary to support these heads would make it unfeasible to employ the head-per-track approach.
Another conventional approach to enhancing access to data divides the cylinders into multiple zones and utilizes multiple actuators and multiple heads per surface. For a given surface, each actuator positions one head only with respect to one of the multiple zones. One example of this approach is illustrated in U.S. Pat. No. 4,577,240. The multi-zone approach is equivalent to providing two separate logical disk drives which can not access each other's data, and does not enhance performance for any one zone of the disk drive.
The multi-zone approach is dictated in part by the use of actuators which support multiple heads in a vertical line which defines a cylinder, each head being associated with a corresponding one of the data surfaces of the disk drive. Because it is generally faster to electronically switch the head which is active than to perform a seek to a new track, the storage area of a disk drive is filled by cylinder. During the recording process, the actuator positions the heads at a selected cylinder and all of the tracks associated with the selected cylinder are filled with data by performing head switches. Then, a seek is performed and data is recorded in the next selected cylinder. The track densities currently in use are such that two sets of heads can not be aligned to have all heads on track for all tracks in a cylinder. This mechanical misalignment presents the same problems as mechanical off-tracking in a single actuator disk drive. Accordingly, a cylinder formatted by one group of heads can not be efficiently used by another group of heads.
A variation on the multiple zone approach is disclosed in the U.S. Pat. Nos. 4,318,145, 4,331,990, and 4,423,448. These patents disclose disk drives having multiple actuator modules, each actuator module having multiple actuators. The actuator modules are arranged so that the head supported and positioned by each actuator primarily services a selected group of tracks and services a second group of tracks as a backup. Any particular track is capable of being serviced by at least two heads. The back-up feature is used in the event that the primary head is inoperative, or to optimize data access by utilizing the back-up head to service some of the tracks which the primary head is intended to service while the primary head is occupied reading data from other tracks. The lack of positioning control between the actuators in the various actuator modules and the imprecise positioning of the heads provided by the linear actuators utilized in this approach makes it difficult if not impossible to provide a high data density or a high performance disk drive suitable for the environment of a work station, personal computer, portable computer, or lap top computer.